Structural foams are used in composite sandwich panels, with the foam sheet sandwiched between opposite outer skins of fibre composite material, to maximise the thickness of the composite sandwich panels, and consequently the panel stiffness, with minimum effect on panel weight. A sandwich panel can be likened to a steel I-beam in which the composite skins act as the I-beam flanges and the core material of structural foam acts as the shear web of the beam. During bending, one skin of the panel is put into compression and the opposite skin is put into tension. This induces a shear loading in the core material. Accordingly, the shear strength and stiffness are critical properties of the core material to prevent excessive deflection and maintain structural integrity.
A high compressive modulus can help prevent localised dimpling from minor impacts and a high compressive strength is often required to withstand loads from through thickness fixings or support points. In larger panels, foam sandwich panels also have to be designed to be resistant to buckling, which can become the major design driver. To prevent localised skin wrinkling, it is important to have a high compressive modulus and high shear modulus to prevent core shear buckling and shear crimping effects. Such composite sandwich panels are used in highly engineered structural members, for example wind turbine blades, that are subjected to demanding mechanical loads in use and so require optimised mechanical properties.
The properties of any polymeric foam core material are highly dependant on the density of the foam, the properties of the base polymer and the microstructure of the cellular arrangement. It is highly desirable to achieve variable properties with fixed foam density and polymer type, i.e. by influencing or modifying the foam microstructure.
The Applicant currently manufactures and sells a structural foam product, known as Corecell (Registered Trade mark). There are several derivatives of the basic foam which are all based on Styrene-Acrylonitrile (SAN) polymer, which provides a unique blend of strength, stiffness and elongation. It is both difficult and very time consuming to modify the polymer composition of the foam in an attempt to modify the mechanical properties of the foam to a selected target.
It is commonly known, however, that inducing anisotropy in the cellular microstructure of a foam can influence mechanical properties. It is known that, the mechanical properties of a foam can vary with cell orientation of the foam.
The anisotropy index (AI) is a numeric parameter that allows characterization of cell orientation of a foam. The knowledge of orientation is very useful because it can be used as an indicator of mechanical properties.
A structural foam is typically made by forming an initial unfoamed embryo which is then transformed into a body of foam in a mould cavity of predefined dimensions. The theoretical AI is calculated from the initial embryo size and the dimensions of the mould cavity.
Various parameters discussed herein are defined as follows:Anisotropy index in the length (or Width) direction=Thickness expansion ratio/Length (or Width) expansion ratio  (1)Thickness expansion ratio=Thickness of foam/Thickness of embryo  (2)Length expansion ratio=Length of foam/Length of embryo  (3)Width expansion ratio=Width of foam/Width of embryo  (4)
When the anisotropy index equals 1, the growth of cells is the same in all directions and their shape is round, i.e. spherical. In this case, the cells are isotropic. When the anisotropy index is below 1, the growth of cells is larger in the length (or width) direction compared to the thickness direction. If the anisotropy index is above 1, the growth of cells is larger in thickness direction compared to length direction (or width direction).
So for a sheet having two major opposite planar surfaces, an AI of greater than 1 indicates greater expansion in the thickness direction of the sheet than in the plane of the sheet, and an AI of less than 1 indicates greater expansion in the plane of the sheet than in the thickness direction.
Most currently available foams exhibit some degree of anisotropy, but this is usually not controlled and is a direct result of the manufacturing process used to produce the foam. Often the foam is isotropic in one plane, for example the plane parallel to the major opposed planar surfaces of a sheet, but exhibits different properties perpendicular to that plane.
A known commercially produced foam sold under the trade name Corecell exhibits an anisotropic index (AI) of 0.85-1.0, which means the cells are elongated in the planar direction of the sheet.
According to theory, by inducing an anisotropic index of greater than 1 (AI >1), in which the cells are elongated in the rise, or thickness, direction, this would improve the mechanical properties of the foam.
Some currently commercially available foams contain a degree of anisotropy, although it is not necessarily known in the state of the art how such foams were specifically manufactured to exhibit such anisotropic properties, or in particular whether or not the anisotropic properties are controllably and reliably introduced or are an “accidental” result of production methods.
Some commercial foams may have AI values greater than 1 but less than 1.4. Such foams can exhibit superior mechanical properties. Foams manufactured by continuous extrusion techniques, often have AI <1, as they tend to be susceptible to cellular alignment in the extrusion direction. It is then possible to bond the extruded sheets together and cut the bonded assembly in a direction perpendicular to the alignment direction. By rotating the axis of the foam, to provide major cut faces of a sheet which are orthogonal to the extrusion direction, this could introduce AI >1 through the thickness of the foam sheet. However, this would be a laborious and expensive manufacturing technique.
One typical current foam manufacturing process involves cooling the expanded foam under an applied pressure within a press apparatus, the cooling/pressing operation occurring after expansion of the foam. This achieves dimensional control of the moulded expanded foam product.
SU-A-1199768 discloses a polystyrene foam production process including preliminary foaming of polystyrene granules, ageing of foamed granules, filling a mould with the granules, moulding them, cooling in the enclosed volume of the mould and extracting the finished product from the mould. The cooling stage is carried out at a mould working volume increased by 10-40% as compared with the initial volume, and after the increase in mould volume, cooling is carried out under a vacuum. JP-A-2004/305708 discloses a foam-moulding method. GB-A-1060908 discloses forming expanded foam articles in an evacuated mould. JP-A-2006/233192 discloses polylactic foamed articles. US-A-2003/236313 discloses making foamed elastomer gels in which various moulding/heating/cooling steps may be with or without a vacuum, as desired. None of these documents discloses the production of anisotropic foams or addresses the problem of producing foams having high shear strength and stiffness, and high compressive modulus and high shear modulus.
The present invention aims at least partially to overcome these problems of known foams and their manufacturing techniques.